Function global_output_source 

fn global_output_source(
    _per_channel: &Arc<FileStore>,
    global: &Option<Arc<FileStore>>,
    wstore: &Arc<Store>,
    block_id: &str,
    filename: &str,
) -> Option<(Arc<FileStore>, String)>

If this channel has no local output for block_id but the agent’s GLOBAL transcript zone (agent:<defId>:current) does, return (global_store, agent_zone). Returns None when the local output is present and non-empty, the block isn’t agent-anchored, there’s no global store, or the global zone is empty — callers then read the per-channel store keyed by block_id.

Only the agent output stream is globalized; every other file stays local. This is what makes opening a cross-channel agent show its conversation: the freshly-opened local block has no output, so the read is transparently served from the global zone the agent wrote elsewhere (mirrored live by the block-controller hot path, or backfilled).